“Safari has a hidden “Debug” menu that offers some additional features for debugging the browser, including stress and load tests, sampling, javascript error logging, the ability to intentionally crash a page, and more. Intended for Safari developers, the Debug menu is different from the Developer menu, which is geared more at web developers, although there are a few options in the Debug menu that general web developers would find useful as well, notably advanced cache choices and CSS animation controls. If this sounds confusing, just enable both of them and poke around and you’ll quickly see the differences.”–via Enable Safari’s Hidden Debug Menu.

defaults write com.apple.Safari IncludeInternalDebugMenu 1